Exclusive measurement of the η→π+π−γ decay

Abstract An exclusive measurement of the decay η → π + π − γ has been performed at the WASA facility at COSY. The η mesons were produced in the fusion reaction pd → He 3 X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13 960 ± 140 events after background subtraction. The measured pion angular distribution is consistent with a relative p -wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of η → π + π − γ . A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the π + π − system.